title: CGG-repeat length threshold for FMR1 RNA pathogenesis in a cellular model for FXTAS.
abstract: Fragile X-associated tremor/ataxia syndrome (FXTAS) is a neurodegenerative disorder that affects carriers of premutation alleles (55-200 CGG repeats) of the fragile X mental retardation 1 (FMR1) gene. The presence of elevated levels of expanded mRNA found in premutation carriers is believed to be the basis for the pathogenesis in FXTAS, but the exact mechanisms by which the mRNA causes toxicity are not known. In particular, it is not clear whether there is a threshold for a CGG-repeat number below which no cellular dysregulation occurs, or whether toxicity depends on mRNA concentration. We have developed a doxycycline-inducible episomal system that allows us to study separately the effects of CGG-repeat number and mRNA concentration (at fixed CGG-repeat length) in neuroblastoma-derived SK cells. Our findings show that there is a CGG-repeat size threshold for toxicity that lies between 62 and 95 CGG repeats. Interestingly, for repeat sizes of 95 CGG and above, there is a clear negative correlation between mRNA concentration and cell viability. Taken together, our results provide evidence for an RNA-toxicity model with primary dependence on CGG-repeat size and secondary dependence on mRNA concentration, thus formally ruling out any simple titration model that operates in the absence of either protein-binding cooperativity or some form of length-dependent RNA structural transition.
authors.affiliation: Department of Biochemistry and Molecular Medicine, University of California, Davis, School of Medicine, Davis, CA 95616, USA.
authors.forename: Gry
authors.initials: G
authors.lastname: Hoem
authors.forename: Christopher R
authors.initials: CR
authors.lastname: Raske
authors.forename: Dolores
authors.initials: D
authors.lastname: Garcia-Arocena
authors.forename: Flora
authors.initials: F
authors.lastname: Tassone
authors.forename: Eleonora
authors.initials: E
authors.lastname: Sanchez
authors.forename: Anna L
authors.initials: AL
authors.lastname: Ludwig
authors.forename: Christine K
authors.initials: CK
authors.lastname: Iwahashi
authors.forename: Madhur
authors.initials: M
authors.lastname: Kumar
authors.forename: Jane E
authors.initials: JE
authors.lastname: Yang
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Hagerman
mesh_terms: D000483:Alleles; D001259:Ataxia / Q000235:genetics* / Q000473:pathology; D002460:Cell Line; D002470:Cell Survival; D004260:DNA Repair; D004318:Doxycycline / Q000494:pharmacology; D051860:Fragile X Messenger Ribonucleoprotein 1 / Q000235:genetics* / Q000378:metabolism; D005600:Fragile X Syndrome / Q000235:genetics* / Q000473:pathology; D015870:Gene Expression; D017930:Genes, Reporter; D005838:Genotype; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D006657:Histones / Q000235:genetics / Q000378:metabolism; D006801:Humans; D034904:Lamin Type A / Q000737:chemistry; D009154:Mutation; D010766:Phosphorylation; D012333:RNA, Messenger / Q000235:genetics; D014162:Transfection; D019680:Trinucleotide Repeat Expansion
chemical_list: D004318:Doxycycline; D051860:Fragile X Messenger Ribonucleoprotein 1; D049452:Green Fluorescent Proteins; D006657:Histones; D034904:Lamin Type A; D012333:RNA, Messenger; C497683:FMR1 protein, human; C468783:H2AX protein, human; C485184:enhanced green fluorescent protein